The present invention relates to methods and apparatus for optical stokes polarimetry, and particularly to fast optical stokes polarimeters and integrated optical Stokes polarimeters.
A rotating waveplate followed by a polarizer acts like a Stokes polarimeter (see, e.g., Collett, Polarized Light: Fundamentals and Applications, Marcel Dekker, Inc. at 102-107 (1993) and Williams, xe2x80x9cRotating-Wave-Plate Stokes Polarimeter for Differential Group Delay Measurements of Polarization-Mode Dispersion,xe2x80x9d Applied Optics, Vol. 38, No. 31 at 6508-6515 (Nov. 1, 1999)). Conventional polarimeters, however, require the physical rotation of the waveplate. Unfortunately, physically rotation is relatively slow because it is limited to the speed of a mechanical actuator (e.g., step motor) used to induce the rotation.
Nonetheless, physical rotation causes the pointing direction of the principal axis of the waveplate to rotate, which changes the way a light beam experiences the birefringence during propagation through the device. As used herein, the xe2x80x9cprincipal axisxe2x80x9d refers to the birefringent axis of a birefringent medium (crystalline or otherwise). Thus, the term xe2x80x9canglexe2x80x9d or xe2x80x9cwaveplate anglexe2x80x9d xcex8 refers to a relative pointing direction of a device""s principal axis with respect to another arbitrary axis, such as an axis fixed with respect to the laboratory reference frame.
Stokes polarimeters can be used to measure the Stokes parameters (i.e., vectors) of a light beam propagating in an optical telecommunications network. These measured parameters can, in turn, be used to calculate the light beam""s state of polarization (hereinafter, xe2x80x9cSOPxe2x80x9d) and degree of polarization (hereinafter, xe2x80x9cDOPxe2x80x9d). Because conventional Stokes polarimeters operate at speeds no greater than 100 kHz, DOP updates are, unfortunately, also limited to these speeds. Moreover, the required tracking and measurement speeds can significantly increase when transmission sources use variable chirped lasers to combat nonlinear effects. Such variations can cause extremely rapid changes in the PSP of an optical fiber due to second order polarization mode dispersion effects. Thus, it may be necessary to measure DOP at rates exceeding 1 MHzxe2x80x94ten times greater than is currently available.
Thus, it would be desirable to provide methods and apparatus for Stokes polarimetry that are fast, reliable, compact, and cost effective.
It would also be desirable to provide methods and apparatus for fast optical distortion compensation, and especially for fast polarization mode dispersion compensation.
It is therefore an object of this invention to provide methods and apparatus for measuring Stokes parameters and other derivable quantities.
It is another object of this invention to provide methods and apparatus for Stokes polarimetry that are fast, reliable, compact, and cost effective.
It is yet another object of this invention to provide methods and apparatus for Stokes polarimetry that are simple to integrate into conventional optical circuits.
It is yet another object of this invention to provide methods and apparatus for fast optical distortion compensation, and especially for fast polarization mode dispersion compensation.
These and other objects are accomplished in accordance with the principles of the present invention by providing methods and apparatus for Stokes polarimetry. A Stokes polarimeter according to this invention can include a rotatable waveplate comprising a variable birefringent material, and a polarizer optically downstream from the waveplate.
An optical distortion compensator according to this invention includes (1) a polarization transformer having an optical input and an optical output, (2) an optical distortion generator having at least one electrical input, an optical input, and an optical output, wherein the generator input is coupled to the transformer output, (3) a Stokes polarimeter having an optical input coupled to the generator output, and (4) a feedback controller having an electrical input and at least one electrical output, wherein said controller input is coupled to said analyzer output and said at least one electrical output is coupled to said generator at least one electrical input.
Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments.